The invention relates to a method of producing a coupling element used for coupling a radiation source in the form of a laser, a semiconductor laser in particular, to a monomode optical transmission fiber by means of a lens.
U.S. Pat. No. 4,143,940 (related to UK Pat. Appl. 76/18,608) discloses a device wherein the end of a flattened monomode optical transmission fiber adjacent to the laser, is covered with a lens formed in situ at the core.
The production of such a coupling element, as disclosed in U.S. Pat. No. 4,143,940, will be described with reference fo FIGS. 1a, 1b and 1c of the drawing of this application. FIG. 1c is a perspective view of a semiconductor laser and an optical transmission fiber. FIG. 1a is a cross-sectional view on line A--A' of FIG. 1c, and FIG. 1b is a cross-sectional view on line B--B'.
In these figures, reference 1 indicates a semiconductor laser. The laser mirrors which are obtained by the semiconductor material-to-air transition are indicated by 9 and 10 respectively. The optical fiber 3 has a sleeve 4 and a core 5. The lens on the head of the flattened core 5 is indicated by 8.
The width of the electric contact 2 ("stripe") on the laser 1 and the thickness of the active layer 11 in the laser more or less determine also the width and the thickness of the working active range of the laser. The width 2 of this range is approximately between 5 and 10 .mu.m and the thickness between 0.5 and 1 .mu.m. As a consequence the cross-section of the emerging laser light is not symmetrical. The divergence of the opening angle perpendicular to the active layer (FIG. 1b) is usually between 40.degree. and 60.degree. and parallel to the active layer (FIG. 1a) only between 10.degree. and 30.degree..
In order to provide in an efficient manner a direct coupling between the laser and the monomode fiber it is necessary to use a cylindrical lens. The coupling efficiency can be increased by slightly flattening the fiber 3 after heating such that the end of the core 5 becomes elliptical. As a consequence the fiber is now adapted to better match the elliptical cross-section of the laser beam in the basic mode. Thereafter, according to the above-referenced patent, a thick layer of negative photoresist is applied on the flattened end face of the fiber 3. After exposure through the core 5 and development there remains a lens 8 which is in the form of a semi-ellipsoid. This lens has a corrective effect on the asymmetrical divergence of the laser beam. This divergence is between approximately 10.degree. and 30.degree. measured parallel to the stripe 2 (FIG. 1a) and between approximately 40.degree. and 60.degree. measured perpendicularly thereto (FIG. 1b).
The method of obtaining the improved result in accordance with the FIGS. 1a, 1b and 1c is rather complicated and there are reasons to doubt a long life of the lens 8.